Download Basic Physical Principles of MRI

Basics of MRI
MRI
1) Put subject in big magnetic field
2) Transmit radio waves into subject [2~10 ms]
3) Turn off radio wave transmitter
4) Receive radio waves re-transmitted by subject0
5) Convert measured RF data to image
Many factors contribute to MR
imaging
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•
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•
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Quantum properties of nuclear spins
Radio frequency (RF) excitation properties
Tissue relaxation properties
Magnetic field strength and gradients
Timing of gradients, RF pulses, and signal
detection
What kinds of nuclei can be used
for NMR?
• Nucleus needs to have 2 properties:
– Spin
– charge
• Nuclei are made of protons and neutrons
– Both have spin ½
– Protons have charge
• Pairs of spins tend to cancel, so only atoms with
an odd number of protons or neutrons have spin
– Good MR nuclei are 1H, 13C, 19F, 23Na, 31P
Hydrogen atoms are best for MRI
• Biological tissues are predominantly 12C, 16O, 1H,
and 14N
• Hydrogen atom is the only major species that is
MR sensitive
• Hydrogen is the most abundant atom in the body
• The majority of hydrogen is in water (H2O)
• Essentially all MRI is hydrogen (proton) imaging
Nuclear Magnetic Resonance Visible Nuclei
A Single Proton
There is electric charge
on the surface of the
proton, thus creating a
small current loop and
generating magnetic
moment µ.
µ
+
+
+
J
The proton also
has mass which
generates an
angular
momentum
J when it is
spinning.
Thus proton “magnet” differs from the magnetic bar in that it
also possesses angular momentum caused by spinning.
The magnetic moment and angular
momentum are vectors lying along the
spin axis
µ =γJ
γ is the gyromagnetic ratio
γ is a constant for a given nucleus
How do protons interact with a
magnetic field?
• Moving (spinning) charged particle
generates its own little magnetic field
– Such particles will tend to line up with external
magnetic field lines (think of iron filings
around a magnet)
• Spinning particles with mass have angular
momentum
– Angular momentum resists attempts to change
the spin orientation (think of a gyroscope)
Ref: www.simplyphysics.com
The energy difference between
the two alignment states depends on
the nucleus
∆ E = 2 µz Bo
∆E = hν
ν = γ/2π Βο
known as Larmor frequency
γ/2π = 42.57 MHz / Tesla for proton
Electromagnetic Radiation Energy
X-Ray, CT
MRI
MRI uses a combination of Magnetic
and Electromagnetic Fields
• NMR measures the net magnetization of atomic nuclei in the
presence of magnetic fields
• Magnetization can be manipulated by changing the magnetic
field environment (static, gradient, and RF fields)
• Static magnetic fields don’t change (< 0.1 ppm / hr):
The main field is static and (nearly) homogeneous
• RF (radio frequency) fields are electromagnetic fields that
oscillate at radio frequencies (tens of millions of times per
second)
• Gradient magnetic fields change gradually over space and can
change quickly over time (thousands of times per second)
Radio Frequency Fields
• RF electromagnetic fields are used to manipulate the
magnetization of specific types of atoms
• This is because some atomic nuclei are sensitive to
magnetic fields and their magnetic properties are tuned to
particular RF frequencies
• Externally applied RF waves can be transmitted into a
subject to perturb those nuclei
• Perturbed nuclei will generate RF signals at the same
frequency – these can be detected coming out of the subject
The Effect of Irradiation to the Spin
System
Lower
Higher
Basic Quantum Mechanics Theory of MR
Spin System After Irradiation
Net magnetization is the
macroscopic measure of many spins
Bo
M
Net magnetization
• Small B0 produces small net magnetization M
• Larger B0 produces larger net magnetization M,
lined up with B0
• Thermal motions try to randomize alignment of
proton magnets
• At room temperature, the population ratio of antiparallel versus parallel protons is roughly 100,000
to 100,006 per Tesla of B0
Recording the MR signal
• Need a receive coil tuned to the same RF
frequency as the exciter coil.
• Measure “free induction decay” of net
magnetization
• Signal oscillates at resonance frequency as net
magnetization vector precesses in space
• Signal amplitude decays as net magnetization
gradually realigns with the magnetic field
• Signal also decays as precessing spins lose
coherence, thus reducing net magnetization
NMR signal decays in time
• T1 relaxation – Flipped nuclei realign with the magnetic
field
• T2 relaxation – Flipped nuclei start off all spinning
together, but quickly become incoherent (out of phase)
• T2* relaxation – Disturbances in magnetic field (magnetic
susceptibility) increase the rate of spin coherence T2
relaxation.
• The total NMR signal is a combination of the total number
of nuclei (proton density), reduced by the T1, T2, and T2*
relaxation components
T2* decay
• Spin coherence is also sensitive to the fact
that the magnetic field is not completely
uniform
• Inhomogeneities in the field cause some
protons to spin at slightly different
frequencies so they lose coherence faster
• Factors that change local magnetic field
(susceptibility) can change T2* decay
Different tissues have different relaxation
times. These relaxation time differences can
be used to generate image contrast.
• T1 - Gray/White matter
• T2 - Tissue/Fluid
• T2* - Susceptibility (functional MRI)
Magnetic Resonance Imaging
• Non-invasive medical imaging method, like
ultrasound and X-ray.
• Clinically used in a wide variety of specialties.
Abdomen
Spine
Heart / Coronary
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Magnetic Resonance Imaging
Advantages:
– Excellent / flexible contrast
– Non-invasive
– No ionizing radiation
Challenges:
– New contrast mechanisms
– Faster imaging
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MRI Systems
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Image Noise and SNR
Low Signal-to-Noise Ratio
High Signal-to-Noise Ratio
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Some Applications of MRI
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Brain / Spine imaging
Knee Imaging
Cardiac Imaging
Whole body scan
Blood vessels (MR Angiography)
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How do I prepare?
• Loose clothing with no metal fasteners.
• Requirement of contrast material
(Gadolium) .
• Radiologists should know about any health
problems or surgeries.
• Pregnant women should not undergo the
MRI exam.
• If you have claustrophobia or anxiety, ask
for a mild sedative.
Contd……
• Accessories can interfere with the magnetic
field of MRI which include:
• Jewellery,watches, credit cards, hearing
aids.
• Pins,hairpins, metal zippers.
• Pens, knives, eyeglasses.
• Metal implants.
Benefits
• MRI is an noninvasive imaging technique
that does not involve any exposure to the
radiation.
• MRI can image soft tissues such as heart.
• It helps physicians to evaluate the structure
and working of an organ.
• Less likely to produce any allergic reaction
than the iodine-based materials used for Xray and CT scans.
Limitations
• High quality images are assured only if you
remain perfectly still.
• An obese person may not fit into opening of
MRI unit ( go for open MRI).
• Presence of an implant hampers the clear
image formation.
• Breathing may cause image distortion.
Contd….
• Not recommended for acutely injured
persons as life support equipment can not be
taken in the area to be imaged.
• Pregnant women are advised not to have an
MRI exam until medically necessary.
Real-Time Interactive MRI
• Shows “live” images.
• Useful when there is motion, such as in the
chest.
• Imaging is very fast, but SNR is lower.
• Interactive imaging allows us to move the
scan plane in real-time.
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The End